How-to guide for slope map of the moon

docs/gallery/images/slope.rst

@@ -0,0 +1,25 @@
+How to make a slope map
+-----------------------
+
+This tutorial show how to make a slope map of the moon from a geographic grid.
+
+We utilize ``grdgradient`` to calculate the gradient magnitude (i.e., dz/dr).
+Since we are working with geographic coordinates (longitude and latitude), 
+it is crucial to set the correct ellipsoid parameter (in this case, for the moon). 
+Finally, we convert this gradient to degrees with ``grdmath``.
+
+.. gmtplot::
+
+   # Make a slope map of the moon.
+   # Calculate gradient magnitude
+   gmt grdgradient @moon_relief_15m -D -Sgradient.nc -fg --PROJ_ELLIPSOID=moon
+   # Convert gradient to degrees
+   gmt grdmath gradient.nc ATAND = moon_slope.nc
+   # Make map with colorbar
+   gmt begin moon png
+    gmt makecpt -Clajolla -T0/12 -I
+       gmt grdimage moon_slope.nc -JW15c -B0
+       gmt colorbar -DJRM+o0.5c/0c+w90%/0.5c+ef -Ba+l"Slope (@.)"
+   gmt end
+   # Delete temporal files
+   rm gradient.nc

docs/index.rst

@@ -58,6 +58,16 @@ Symbols and markers
 Images, contours, and fields
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. grid:: 2 3 3 4
+
+    .. grid-item-card:: :doc:`gallery/images/slope`
+       :padding: 1
+       :link-type: doc
+       :link: gallery/images/slope
+
+       .. image:: _images/6b12ba3c82f97e0841efb186eef0aa40.png
+
+
 3d plots
 ~~~~~~~~